"I love [Rihanna] and every time I see her, I'm reminded of the light that she has," Perry explained to Elle UK. "There's a lot of dark in this business. I know a lot of people out there with the most detrimental entourages -- they are the root of their demise."

"It's really unfortunate but you can't save these people. My days of celebrity saving are over."

And Katy wanted to make sure that Stewart -- also a buddy -- knew that.

"I sent [Kristen] a text message saying, 'I know you've seen all this stuff but you know I would never disrespect you. I'm not that person. I'm just trying to be a friend to him but it is unfortunate that I do have a set of tits.'"

You can check out the full interview with Perry in the September issue of Elle UK on newsstands tomorrow, July 31. (July is the new September, you guys.)